vfs: add ENAMETOOLONG checks to path helpers in fs.c

vfs: add ENAMETOOLONG checks to path helpers in fs.c

Implement L1: standardize ENAMETOOLONG across all path helpers.

Added path length checks (PATH_MAX = 128) to:
- vfs_lookup_depth(): Check path length before lookup
- vfs_lookup_parent(): Check path length before parent lookup
- vfs_mkdirp(): Check path length before recursive mkdir

These functions now return NULL or -ENAMETOOLONG when paths exceed
128 characters, matching the behavior already present in syscall.c
helpers (copy_user_cstr, path_resolve_user).

Test: make test-battery PASS (157/157), make analyzer PASS